Week 6 - Data Transmission II.pdf

# CSC 1104 Data Communications and Networking ## Lecture 6 - Data Transmission II ### Data Transfer Technique There are 2 different types of switching networks used to transfer data from source to destination: - **Circuit Switching Network** In a circuit switching network, a dedicated path from...

# CSC 1104 Data Communications and Networking ## Lecture 6 - Data Transmission II ### Data Transfer Technique There are 2 different types of switching networks used to transfer data from source to destination: - **Circuit Switching Network** In a circuit switching network, a dedicated path from source to destination must be established before data is transmitted. This path is dedicated to the source-destination pair for the duration of the communication session. Eg: Telephone Networks - **Packet Switching Network** In a packet switching network, data is organized in blocks called packets, which are then sent out. A packet switching network does not need a dedicated path to be established before data is transmitted. Eg: Internet **Ibe, O.C., FUNDAMENTALS OF DATA COMMUNICATIONS NETWORKS, John Wiley & Sons, Inc, 2018** ### Overview of delay in packet switched network A diagram illustrates the delay in a packet switched network. A node delays a packet by: - Processing delay - Read packet header - Determining the outgoing link - Check bit errors - Value:μs - Queuing delay - Wait to be transmitted - Value: ms - Transmission delay - Pushing all bits of the packet into the link - $d_{trans}=\frac{L}{R}$ - Value: ms - Propagation delay - Time taken for data to travel from one router to another - Speed based on propagation speed of physical media, *s* - Distance between the routers, *d* - $d_{prop}=\frac{d}{s}$ - $s= 3 \times 10^8$ meters/sec $d_{node}=d_{proc}+d_{queue}+d_{trans}+d_{prop}$ ### Signal Encoding Techniques The document describes how to change the format of data signals for different applications. - **Digital Data, Digital Signals** - **Analog Data, Digital Signals** - PCM (Pulse coded modulation) - PAM (Pulse Amplitude Modulation) - PWM (Pulse width modulation) - **Digital Data, Analog Signals** - ASK (Amplitude shift keying) - FSK (Frequency shift keying) - PSK (Phase shift keying) - **Analog Data, Analog Signals** - AM (Amplitude modulation) - FM (Frequency modulation) - PM (Phase modulation) ### Analog and Digital Signaling A diagram illustrates how to encode data to a digital signal(a) and how to modulate a digital signal onto an analog signal(b) - **Digital Signaling** Takes a signal, *g(t)* that is either digital or analog, and encodes it to *x(t)* which is a digital signal. - **Analog Signaling** Takes a signal, *m(t)*, which is either digital or analog, and modulates it to *s(t)*, an analog signal. ### Digital-to-Digital Conversion Digital-to-digital conversion refers to representing digital data by using digital signals There are three techniques involved: - **Line Coding:** Line coding is always needed - **Block Coding:** May or may not be needed - **Scrambling:** May or may not be needed ### Line Coding and Decoding A diagram illustrates how a sender encodes digital data into a digital signal, which is transported over a link before being decoded by the receiver to retrieve the original data. - The sender encodes the data in a digital format. - The encoded signal is transmitted over a link to the receiver. - The receiver decodes the signal to retrieve the original digital data. ### Signal Element Versus Data Element The document discusses the relationship between signal elements and data elements in data transmission. - The data rate defines the number of data elements (bits) sent in 1 second. The unit is bits per second (bps). - The signal rate is the number of signal elements sent in 1 second. The unit is the baud. - The goal in data communications is to increase the data rate while decreasing the signal rate. Increasing the data rate increases the speed of transmission. Decreasing the signal rate decreases the bandwidth requirement. - $S = c \times N \times \frac{1}{r}$ ### Line Coding A diagram shows the different line coding strategies. - **Unipolar** The signal levels are on one side of the time axis. For example, a positive voltage represents a binary 1, and an idle line represents a binary 0. - **Polar:** The voltages are on both sides of the time axis. For example, the voltage level for 0 can be positive, and the voltage level for 1 can be negative. - **Bipolar:** In bipolar encoding, there are three voltage levels: positive, negative, and zero. ### Unipolar NRZ Scheme A diagram illustrates the Unipolar NRZ scheme. - Unipolar line coding is a very basic method of encoding and is rarely used with most line encoding being either polar or bipolar or variations of these. - It is not self-clocking, and it has a significant DC component. ### Polar NRZ-L and NRZ-I Schemes A diagram illustrates the NRZ-L and NRZ-I schemes. - In NRZ-L, the level of the voltage determines the value of the bit. - In NRZ-I, the inversion or the lack of inversion determines the value of the bit. - NRZ-L and NRZ-I both have an average signal rate of N/2 Baud. - NRZ-L and NRZ-I both have a DC component problem. - They lack synchronization capability. ### Polar Biphase: Manchester A diagram illustrates the Manchester and Differential Manchester schemes. - In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization. - The minimum bandwidth of Manchester and differential Manchester is two times that of NRZ. - Midbit transition serves as a clocking. ### Bipolar Schemes:AMI and Pseudoternary A diagram illustrates the AMI and Pseudoternary schemes. - In bipolar encoding, we use three levels: positive, zero, and negative. ### Synchronization A diagram illustrates the synchronization of data over a transmission link. - If the receiver clock is faster or slower than the sender's clock, the bit intervals are not matched, and the receiver might misinterpret the signals. - A self-synchronizing digital signal includes timing information in the data being transmitted. Transitions in the signal can alert the receiver to the beginning, middle, or end of the pulse. ### Block Coding A diagram shows how to encode data using block coding. - Block coding provides redundancy to ensure synchronization and to provide inherent error detection. - In general, block coding changes a block of *m* bits into a block of *n* bits, where *n* is larger than *m*. - Block coding is referred to as an *mB/nB* encoding technique. - Block coding normally involves three steps: division , substitution, and combination. - In the division step, a sequence of bits is divided into groups of *m* bits. - For example, in **4B/5B** encoding, the original bit sequence is divided into 4-bit groups. - The heart of block coding is the substitution step. In this step, a *m*-bit group is substituted with an *n*-bit group. - For example, in **4B/5B** encoding, a 4-bit group is substituted with a 5-bit group. - Finally, the *n*-bit groups are combined to form a stream. ### Block Coding Table A table shows how different data sequences are converted to encoded sequences, including control sequences. - If a 5-bit group arrives that belongs to the unused portion of the table, the receiver knows that there has been an error in the transmission. ### Transmission Modes: Parallel VS Serial Transmission - Binary data can be organized into groups of *n* bits each. Computers produce and consume data in groups of bits rather than letters. This is called parallel transmission. - The mechanism for parallel transmission is simple: use *n* wires to send *n* bits at one time, so that each bit has its own wire, and all *n* bits of one group can be transmitted with each clock tick from one device to another. - The advantage of parallel transmission is speed. It can increase the transfer speed by a factor of *n* over serial transmission. - The significant disadvantage of parallel transmission is cost. The cost for communication lines, *n* in this case, can be high. - Because of the cost, parallel transmission is usually limited to short distances. ### Transmission Modes: Asynchronous and Synchronous - When two devices exchange data, the data flows between the devices as a continuous stream of bits. - There are two basic transmission techniques for separating the groups of bits: asynchronous transmission and synchronous transmission. - These methods are necessary for devices to know when a byte begins or ends. ### Asynchronous transmission - Asynchronous transmission transmits one byte at a time over a line at random intervals. Each byte is framed by controls: a start bit for marking the beginning of the byte, a stop bit for marking the end of the byte, and a parity bit for error checking. This is relatively slow and used for low-speed transmission. - A diagram shows the time intervals of the transmission of a byte. ### Synchronous transmission - Synchronous transmission transmits groups of bytes simultaneously at regular intervals. The beginning and ending of a block of bytes are determined by the timing circuitry of the sending and receiving devices. This provides much higher speeds and greater accuracy than asynchronous transmission. - "Some people use the term synchronous to refer to real-time live communications and the term asynchronous to refer to communications that are not real time." - A diagram shows a block of bytes transmitted synchronously. ### Analog to Analog Transmission - Analog-to-analog conversion refers to the representation of analog information by an analog signal. - This is done by modulation. - An example: FM radio and AM radio, TV signal transmission. ### Filters - A filter or an electrical filter is used to modify, reshape, or reject unwanted frequencies of an electrical signal. - Filters accept and pass only those signals wanted by the circuit designer. - The most common filter types are: - **Low-pass filter:** Allows low-frequency signals from 0Hz to the upper cutoff frequency (*f*) and blocks all frequencies higher than *f*. - **High-pass filter:** Allows high-frequency signals from cutoff frequency (*f*) and blocks all frequencies lower than *f*. - **Band-pass filter:** Allows signals falling between the frequency band from lower cutoff frequency (*f*) to upper cutoff frequency (*f*) and blocks all frequencies lower than *f* and higher than *f*. - **Band-stop filter:** Allows signals falling outside the frequency band from upper cutoff frequency (*f*) to lower cutoff frequency (*f*) and blocks all within (*f*, *f*). - A diagram shows the responses of each filter type. ### Baseband VS Passband Transmission - Baseband signals: - Voice (0-4kHz) - TV (0-6 MHz) - A signal can be sent in its baseband format when a dedicated wired channel is available. - Otherwise, it must be converted to passband. ### Modulation: What and Why? - The process of shifting the baseband signal to passband range is called Modulation. - The process of shifting the passband signal to baseband frequency range is called Demodulation. - The wavelength is reduced for efficient transmission and reception. The optimum antenna size is ¼ of a wavelength. - A typical audio frequency of 3000 Hz has a wavelength of 100 km and would need an effective antenna length of 25 km! By comparison, a typical FM carrier is 100 MHz, with a wavelength of 3 meters. - Simultaneous use of the same channel (called multiplexing) is made possible. Each unique message signal has a different assigned carrier frequency (e.g., radio stations) and share the same channel. The telephone company invented modulation to allow phone conversations to be transmitted over common phone lines ### Modulation - Carrier frequency or carrier signal is a high-frequency signal produced by the sending device that acts as a base for information transfer. - The receiving device is tuned to the frequency of the carrier signal it expects from the sender. - The transmitted data then changes the carrier signal by changing one or more of its characteristics (amplitude, frequency, phase). - This modification is called modulation. - The modulation types are classified into three categories: - Amplitude modulation - Frequency modulation - Phase modulation ### Amplitude Modulation (AM) - In amplitude modulation (AM) transmission, the carrier signal is modulated so that the amplitude varies with the changing amplitudes of the modulating signal. - Frequency and Phase of the carrier signal remain the same. - Changing amplitude follows the variations in the information. - AM Bandwidth: - This modulation creates a bandwidth that is twice the bandwidth of the modulated signal. - The bandwidth created covers a range centered on the carrier frequency. - AM stations are separated by 10kHz to prevent overlapping of signals. - AM bandwidth is 500-1700 kHz. - A diagram illustrates AM modulation. ### AM Band Allocation - The total bandwidth required for AM can be determined from the bandwidth of the audio signal: - $B_{AM}=2B$ - The bandwidth of an audio signal (speech and music) is usually 5 kHz. - Therefore, an AM radio station needs a bandwidthof 10kHz. This fact is supported by the Federal Communications Commission (FCC), which allows 10 kHz for each AM station. - AM stations are allowed carrier frequencies anywhere between 530 and 1700 kHz. - A diagram illustrates the band allocation for AM radio. ### Frequency Modulation (FM) - In frequency modulation (FM) transmission, the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal. - Peak Amplitude and Phase of the carrier signal remain the same. - As the amplitude of the information signal changes, the frequency of the carrier signal changes correspondingly. - FM Bandwidth: - FM stations are separated by 200kHz to prevent overlapping of signals. - FCC requires that alternate bandwidth allocations are used due to privacy. - FM bandwidth is 88-108 MHz. - A diagram illustrates FM modulation. ### FM Band Allocation - The total bandwidth required for FM can be determined from the bandwidth of the audio signal. - The bandwidth of an audio signal (speech and music) broadcast in stereo is almost 15 kHz. - The FCC allows 200 kHz (0.2 MHz) for each station. - Some extra guard band is also added to avoid interference. - FM stations are allowed carrier frequencies anywhere between 88 and 108 MHz. Stations must be separated by at least 200 kHz to keep their bandwidths from overlapping. - A diagram illustrates the band allocation for FM radio. ### Phase Modulation (PM) - In phase modulation (PM) transmission, the phase of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal. - Peak Amplitude and frequency of the carrier signal remain the same. - As the amplitude of the information signal changes, the phase of the carrier signal changes correspondingly. - PM Bandwidth: - The bandwidth of PM signals is difficult to determine exactly. - A diagram illustrates PM modulation. ### Amplitude, Phase, and Frequency Modulation of a Sine-Wave Carrier by Sine-Wave Signal - A diagram illustrates how amplitude, phase, and frequency modulation affect the carrier signal. The carrier signal is modulated by a sine-wave signal. ### Multiplexing - A technology is improving. More data is being transmitted constantly. To prevent bandwidth of transmission medium overload, we are constantly creating more transmission links, increasing bandwidth. - But, when the bandwidth of the transmission medium link between 2 devices is more than what is needed, what happens then? - To prevent wastage of bandwidth, the link can be shared by multiplexing. - Multiplexing is a set of techniques that allow simultaneous transmission of multiple signals across a single link. - In a multiplexed system, one link can have *n* number of channels. Multiple transmission streams to a multiplexer. - The multiplexer combines them into a single stream. At the receiving point, the multiplexed stream passes through a demultiplexer to separate the stream into its component transmission. - A diagram shows the working of a multiplexer and demultiplexer. - There are three basic multiplexing techniques: - Frequency-division multiplexing (FDM): For analog signals. - Wavelength-division multiplexing (WDM): For analog signals. - Time-division multiplexing (TDM): For digital signals. - A diagram shows the three techniques and their corresponding signal types. ### Frequency-division Multiplexing (FDM) Frequence-division multiplexing is an analog multiplexing technique applied when the bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals being transmitted. - Signals generated by each sending device modulate different carrier frequencies. - The signals are then combined into a single composite signal that is transported through the link. - A diagram shows the FDM process. ### Multiplexing Process - During multiplexing, each source generates a signal of a similar frequency range. These signals are modulated onto their corresponding carrier frequency (*f1*, *f2*, *f3*). The resulting modulated signals are then combined to form a single composite signal that is sent out through the link. - A diagram shows the process of multiplexing. ### Demultiplexing Process - On arrival at the receiving end, the composite signal passes through a demultiplexer. - It is a series of filters to decompose the multiplexed signal into its own constituent component signals. - The individual signals pass through a demodulator that separates them from their carrier signal to get the information signal. - A diagram shows the process of demultiplexing. ### Frequency-Division Multiplexing (FDM) - Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. - A diagram shows the configuration using the frequency domain, assuming there are no guard bands. ### Frequency-Division Multiplexing (FDM) - A diagram illustrates how three voice channels are combined into a single link with a wider bandwidth. ### Frequency-Division Multiplexing (FDM) - Five channels, each with a 100-kHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there needs to be a guard band of 10 kHz between the channels to prevent interference? - For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5*100+4*10 = 540 kHz. ### Frequency-Division Multiplexing (FDM) - A diagram illustrated how to combine five channels with a 100-kHz bandwidth together. ### Wavelength-Division Multiplexing (WDM) - Wavelength-division multiplexing (WDM) is fundamentally similar to FDM. - WDM is designed to use the high-data-rate capability of fibre-optic cable. - The data rate of fibre-optic cables is high, and using it for just one signal is a waste. - The combination of multiple signals at different frequencies allows for a higher data transmission rate. - In WDM, the multiplexer combines several light sources into one and sends them through the link. - It is then demultiplexed at the receiving site into its corresponding light signal. - A diagram shows the process of WDM. ### Time-Division Multiplexing (TDM) - Time-division multiplexing (TDM) is a digital process that allows several connections to share the high bandwidth of a link. - Instead of sharing a portion of bandwidth, time is shared in TDM. - Each connection occupies a portion of time in the link, and transmission happens sequentially. - A diagram illustrates the process of TDM.